The present invention relates to pumps and compressors and in particular to balancing piston drive assemblies in such devices.
One problem with conventional pumps and compressors is that they can create excessive noise and vibration as the piston(s) are reciprocated. The piston is typically eccentrically mounted to the drive shaft so that it travels back and forth in the stroke direction as it reciprocates in the cylinder. It is also weighted more heavily at the head where compression occurs. Thus, the force acting on the drive shaft is ordinarily not balanced about the axis of the drive shaft. Moreover, as the piston reciprocates between the pump and suction strokes, the force on the drive shaft changes.
To reduce this unbalance, the end of the piston opposite the head can be weighted. By adding the appropriate mass to the piston at a side of the drive shaft opposite the piston head, essentially equal and opposite oscillatory forces can be achieved with respect to the drive shaft. German publication DE 4122198 A1 discloses a counter weighted piston arrangement. Another German publication, DE 19903185 C2 discloses a drive assembly in which the loading on the drive shaft is even more in balance. Here, the oscillatory forces of the piston are balanced about an axis of the eccentric by a counter weight mounted to the lower end of the piston. Then, these forces centered on the eccentric are balanced about the drive shaft by another counter weight which is mounted to the drive shaft to rotate therewith. The rotating weight is concentric with the drive axis but is weighted to offset the forces at the eccentric axis which revolve about the drive axis as the drive shaft turns. With this assembly, therefore, the oscillatory and rotational forces are essentially balanced about the drive shaft axis. The entire disclosure of these publications are incorporated by reference as though fully set forth herein.
Even the improved balancing taught by the latter German reference, however, does not address the loading on the drive shaft that occurs in the direction of the drive axis. The axial distance between the force centered on the eccentric and the countering force at the rotating weight creates a moment, or shaking couple, on the drive shaft in the axial direction, such that unwanted vibration, and the corresponding noise and inefficiency, can remain.
This type of unbalancing is common in single cylinder pumps, however, it also occurs in multi-cylinder pumps. Even in opposed piston pumps where the pistons are 180° out of phase, the forces on the drive shaft can be unbalanced and cause the drive assembly to vibrate. The pistons are typically coupled to the drive shaft spaced apart along the drive axis, for example using a single retainer or eccentric element between the connecting rods of the pistons. Thus, a moment, or shaking couple, arises as the drive shaft rotates because of the axial spacing between the pistons even if they are otherwise balanced individually.
Accordingly, a piston drive assembly with better vibration dampening characteristics is needed.